Electronic regenerative repeater



Dec. 17, 1957 K. WHEELER ETAL ELECTRONIC REGENERATIVE REPEATER 2 Sheets-Sheet 1 Filed July 8, 1949 Dec. 17, 1957 L. K. WHEELER ETAL ELECTRONIC REGENERATIVE REPEATER 2 Sheets-Sheet 2 Filed July 8, 1949 Leandr/ A4 l/kee/ec 5 /l/frea C. Frosf,

A, XM M United States Patent i ELECTRONIC REGENERATIVE REPEATER Leonard Keith Wheeler and Alfred Cecil Frost, London,

England, assignors to Her Majestys Postmaster General, London, England Application July 8, 1949, Serial No. 103,728

Claims priority, application Great Britain October 15, 1948 Claims. (Cl. 178-70) This invention relates to electronic regenerative repeaters for start-stop telegraph systems.

An object of the invention is to provide an improved electronic regenerative repeater which, while employing a comparatively small number of valves, is capable of being arranged to satisfy a number of operational requirements.

When characters of a code are transmitted by start-stop telegraphy the timing of each train of signals associated with a character is independent of the timing of any other character and a fresh character may be transmitted at any arbitrarily determined time after a preceding character has been transmitted. In order to enable this to be done the code signals representing each character are preceded by a start signal which is of the same polarity (known as space) for all characters and are followed by a stop signal which is also of the same polarity (known as mark) for all characters. Commonly used codes have five or siX equal length elements which may be of mark or space polarity in various permutations to represent various characters. The length of the start signal is usually equal to that of a code element and in various systems the stop signal is from 1 to 1 /2 times the length of a code element.

Due to well-known causes signals are liableto distortion when transmitted and this results in an alteration in the time interval between the instant when the start signal of a character is commenced and the instant at which an element of the code is commenced and/ or completed. In order to overcome the well-known effects of such variations of the timing of the instants of modulation regenerative repeaters are employed which sample incoming signals at the nominal mid-points of the time lengths of each of the unit elements. According to whether the signal at each mid-instant is space or mark the repeater transmits an element equal in time duration to the nominal period of the transmitted element. In signalling codes when the stop signal differs in length from the code elements it is usual to sample the stop signal 0.5 of a code element time length after its nominal instant of commencement.

Mechanical repeaters, owing to the inertia of the mov ing parts, do not function with suificient speed to enable sampling to be efiected substantially instantaneously and the time taken may be an appreciable fraction of a signal element time length. This limits the maximum distortion which can be tolerated whilst still permitting correct regeneration of signals.

2,816,956 Patented Dec. 17, 1957 Electronic repeaters have therefore been proposed and such repeaters enable the sampling of an incoming element to be carried out more nearly instantaneously.

The improved electronic regenerative repeater according to the invention comprises an electrical circuit adapted to receive' signals from an incoming line and to control the operation of means for applying signals of polarity corresponding to the polarity of the received signals to an outgoing line, impulse applying means for applying impulses to the said circuit and thereby to place said circuit into condition to control the operation of said signal applying means and means for rendering said impulse applying means operative to apply said impulses immediately a start signal has persisted on the incoming line for a predetermined minimum period.

According to a further feature thereof the invention provides an oscillator for applying a sequence of spaced impulses to the circuit which is adapted to control the operation of the signal applying means.

Should a faulty or distorted incoming signal be the stop signal, the receiver or repeater as the case may be may not be restored to the rest condition at the conclusion of the transmission of a character with the result that the receiver or repeater may immediately commence a further cycle of operations and record or transmit in succession a number of false characters. The reception of real characters interspersed with such false characters would obviously result in confusion. The repeater according to a further feature of the invention provides means for the automatic application of a stop signal to the outgoing line so that an idle period is ensured between succeeding characters.

Difficulties may also arise, on both radio and line links, due to the appearance of short spurious periods of spacing or start signal polarity, which periods may last for only a few milli-seconds. If one of the spurious periods occurs during the transmission of the code elements of a character an error may be produced but only if it coincides with an instant of sampling or selection. If, however, such spurious start signal occurs during an inter- 'val between successive characters, the receiving device may commence a cycle of operations and if the next genuine character starts to arrive during the cycle, the device may get out of step with the signals. Even during an idle period if a succession of short spaces arrive, spurious characters may be transmitted. No special measures need in general, be provided in the construction of a receiver such as an electromechanical teleprinter to guard against these efiects as the inertia of the receiving electromagnet makes it unresponsive to very short signals. In the case of an electronic receiver, however,.such as is used in an electronic repeater, unless some device is specially provided to guard against it, the appearance of the spurious short start signal will produce errors of transmission. Space or start signals longer than 0.5 of an element length must not be rejected as such signals are probably genuine start signals but signals of less. than this length may reasonably berejected when, as in the application of a feature of the present invention means are provided for preventing the repeater. from being started until a start signal has persisted for a predetermined period. A further feature is that the meanswhichpredeterminesthe period is sensibly immediately restored to its initial condition if the start signal is spurious and does not persist for the predetermined period.

In switched printing telegraph networks it is often required to use a long (longer than one character) space signal to clear down a circuit connection. The use of a repeater employing simple stop-signal insertion would prevent this facility being available because an incoming long space would result in all space characters with inserted stop signals being transmitted. The invention, therefore, in a still further feature thereof, provides means for suppressing the automatic application of a stop signal when an all space character is being received.

The repeater arrangedwith this additional-feature can still read andfretransmit-astop signal ;if;it be-present even where the code employed normally; makesuse of ,an all space" character and thus, with suitable, design, both long space transmissionand. automatic.stop vv signal insertion may be provided in arepeateryat the sametirneialthough stop signals will not be automatically.insertediat the end of all space characters.

The repeater provided bytheinvention, as will-bedescribed, makes it possible to provide; that the minimum length of the stop-signal is the mostsuitable in thesystem employed.

The invention will be described by.way otexarnple with reference to the accompanying drawings,. the-Figures;l and 2 of which together show. diagrammatically;thecircuit arrangements of an electronic regenerative repeater embodying the invention.

In the drawing various circuit arrangements are shown grouped in rectangles from and .towhich .leads,;marked with arrows, are shown to indicate the inter-connection of the various groups of circuits and to show the direction in which electrical currents or voltages pass to control the operation of the circuit which receives them. These interconnecting leads terminate at the boundaries of, the rectangles. In order to show that..the interconnecting leads are continuations of certainleads which commence within the rectangles, the latter leadsand-the interconnecting leads are marked with-identical reference letters.

Referring to the drawings teleg ap hic signals; are received by the repeaterover a transmission; line X'and the regenerated correct signals areretransrnitted by the repeater over the output line Y to, for example, another repeater or a teleprinter apparatus. Theincoming line X divides into two branches Aand A branch;A-leading-,to

a signal receiving circuit SEC where theincoming signals are examined and ,branch A itself being diVidedinto two sub-branches Q and P, sub-branch Q leadingg to: a start gate and stop-signal. suppressor circuit. SGSS1and; subbranch P to a start delay circuit SDC,

The above circuitand other circuits .whi ch are. included:

in the receiver are, in the. particularexamplenow described, arranged to be connected with high-tension supply sources of 150 volts. and-150 volts .while the line-signals are :80 volts. Suitable circuit component values for use with those voltagesaregiven below but .itwill be understood that thesecornponent values may -be altered should different supply and signalvoltagesabe employed while valves other than, those specif ed .may. also,-require the component values to bedifferentfrom those specified. The nature of the changes will be apparent to those :skilled in the art. 7

The repeater now being described is arranged tooperate a 7 /2 unit start-stop. telegraphic .code:at;,a-speed-ch50 bauds but it willv beappreciated that the repeater;is;capable of being applied, to, otherv equal character-length.

start-stop codes and. speeds. ofltransmissionqby; suitable-- selection of the valuesof thevarious components.

In the following-description timeis indicated inmilliseconds from the commencement of; the received start signal. Start signal polarity islcalled.sp ace.2 and the line condition for a start signal is positive. Stop signalvpolan ity is called mark and the line condition for a stop signal is negative.

The incoming line signal is applied over branch A to the receiving circuit SEC which comprises a difierential gate circuit to which sampling or examining impulses are applied. The circuit SEC comprises a multi-grid (pentode) valve V3 and the branch A is connected to the suppressor grid of this valve through resistor R1. The screen grid of valve V3 is connected to an outgoing lead L via capacitor C2 and the control grid of the valve is connected to incoming lead J. The anode of valve V3 is connected to an outgoing lead K via capacitor C1. The suppressor grid is connected to earth via resistor R2 and the control grid is normally negatively biassed to cut off anode current. The screen-grid is also connected, via potential divider R4, R5 and capacitor C3 to outgoing lead H. Incoming lead I is arranged to carry positive impulseswhichare muchshorterin durationthan aunit signal element tothe control grid so that valve V3 is conditionedto conductover eitherthe screen grid or boththe screen grid'and anode. If'the incoming signal'is amark the suppressor grid is held negative and the anode cannot conduct. The screen grid however momentarily conducts when the positive impulse is applied over lead I and a negative impulse is sent over lead L. Should the incoming signal be space both the screen grid and the anode can conduct when the positive impulse is applied over lead I as the suppressor grid potential is raised by the incoming space signal. Negative impulses are thus sent over both leads L and K. The positive impulses are applied over lead I at times t=10 ms., t=30 ms., t=l30 ms. so that the negative impulses appear over lead L or leads L and'K at these instants. The relative values of capacitors C1 and C2 are selected so that a negative impulse over lead K persists slightly longer than that over lead L.

Negative impulses are also sent over lead H at the same time as they are sent over lead L.

Leads L and K are connected to an output trigger circuit OTC which includes two valves V1 and V2 arranged to form an Eccles Jordan trigger circuit in which the valves have two stable states, i. e. in one stable condition of the trigger valve V1 conducts and V2 is non-conducting and in the other stable condition valve V2 conducts while valve V1 is non-conducting. Both valves are multi-grid (pentode) valves and the lead L is connected to the suppressor grid of valve V2 while lead K is connected to the suppressor grid of valve V1. The anode of valve V1 is connected via a resistor to the control grid of valve V2 and the anode of valve V2 is connected via resistor R7 to the control grid of valve V1. An incoming lead N is connected via resistor R6 to the control grid side of resistor R7 and negative bias is applied to the control grid of valve V1 .via resistor R8. Negative bias is applied in similar manner to the control grid of valve V2. Resistors R6, R7 and R8 form a potential divider network which is so proportioned that when lead N, the potential of which varies from a maximum to a minimum value, is at the lower end of its potential excursion the trigger circuit has two stable conditions and will respond to the impulses over leads L and K- but when lead N is-at the positive end of its potential excursion the control grid of valve V1 is held positive and V1 is maintained in the conducting state.

In the condition when lead N is at the lower end of its potential excursion a negative impulse overlead L from circuit SEC, with no impulse over lead K, will cut-off valve V2 at the suppressor grid, so that-the control grid of valve V1.is held positive and this valve con ducts over its anode.

When impulses appear simultaneously over leads L and K'both valves V1 and V2 are cut oil for a short time at their suppressor gridsbut as the impulse over lead K is the longer, valve V1 only is eventually cut oil? and .valve V2 conducts.

Anode leads M and S of valves V1 and V2'respectively are conducted to circuit OPR whichcomprises the windings PR and PR of a polarised relay, the armature PR of which is connected to outgoing line Y and is arranged to apply positive (space) polarity or negative (mark) polarity to the line Y according to its setting.

As mark potential on the incoming line X causes the valve V3 of circuit SEC to be cut off at its suppressor grid, when a positive impulse is applied over lead I the consequent impulse sent over line L causes valve V1 to conduct over its anode circuit and mark potential to be applied to the outgoing line Y by the relay OPR. Space potential on the incoming line X causes the impulse over lead K to preponderate and valve V2 to conduct whereupon relay OPR is operated to apply space potential to the outgoing line Y.

When a positive voltage is applied over lead N the valve V1 is caused to conduct and mark potential applied to the outgoing line Y whatever the condition of the signals on the incoming line X.

The positive impulses over lead J are generated by a multivibrator MVB which comprises two valves V7 and V8 arranged as a relaxation oscillator so that each valve is adapted to alternate between conducting and \non-conducting states, one valve conducting while the other is non-conducting. The valves V7 and V8 are multi-grid (pentode) valves with their control grids connected to the H. T. supply through resistors R10 and R9 respectively. The anode of valve V7 is coupled through capacitor C4 with the control grid of valve V8 and the anode of valve V8 coupled with the control grid of valve V7 through capacitor C5. The suppressor grid of valve V8 is connected to the cathode of this valve and to earth. The screen grid of valve V8 is connected to lead J through capacitor J The suppressor grid of valve V7 is connected to incoming lead G and to a diode valve V6 which is connected across this suppressor grid and earth so that the potential of this grid cannot rise appreciably above earth potential when a positive potential is applied to lead G.

When no signals are being transmitted over the incoming line X the potential of line G is negative and the valve V7 is held out off at its suppressor grid. The multivibrator MVB is then inoperative and incapable of transmitting impulses over the lead I. When, however, a positive potential is applied to lead G, the suppressor grid of valve V7 reaches earth potential and, due to i the positive voltage applied to its grid through resistor R10, valve V7 conducts thus generating a negative impulse at its anode. This negative impulse is applied to the control grid of valve V8 and this valve becomes nonconducting with the result that a positive voltage is applied to the control grid of valve V7. When valve V8 becomes non-conducting a positive impulse is transmitted over lead I to the control grid of valve V3 of circuit SEC which then operates as described above.

The negative voltage applied by valve V7 to the control grid of valve V8 dies away and valve V8 then conducts, consequently applying a negative voltage to the control grid of valve V7. In this way the valves V7 and V8 alternately become conducting and non-conducting. The period of oscillation of the oscillator thus formed by valves V7 and V8 is mainly controlled by the values of resistors R9 and R10 and of capacitors C4 and C5. Resistors R9 and R10 are tapped so that accurate adjustment of the period can be made. In the example described the period is milliseconds and produces at lead I impulses at t=l0 ms., t= ms., t=130 ms. When the oscillator has transmitted a pulse at time t=130 ms., it is stopped by removal of the positive potential from line G. The multivibrator MVB is set in operation immediately after a start signal on the incoming line has persisted for a predetermined minimum period, in the example described 10 milliseconds, or in other start signal.

The arrangement by which the control grids of valves V7 and V8 are connected to H. T. supply has the advantage that by it the behaviour of the oscillator is less affected by variations in valve characteristics.

The multivibrator MVB is set in operation and stopped by a timing circuit MVC which determines the potential of the lead G. The circuit MVC comprises a multi-grid (pentode) valve V4, the suppressor grid of which is connected to an incoming lead D. The anode of valve V4 is connected via resistor R17 with the control grid of a multi-grid (pentode) valve V5, while the control grid of valve V4 is connected via capacitor C6 with the anode of valve V5, the two valves being arranged in a pulse trigger circuit. Positive potential is applied on the control grid of valve V4 via a resistor R19, the values of R19 and C6 being so determined that the natural relaxation period of the trigger is slightly greater than milliseconds but such that a negative impulse applied to the control grid of valve V5 after the period of 120 milliseconds has elapsed will cause the valve V5 to become non-conducting.

The suppressor grid of valve V5 is connected to cathode and earth while the screen grid of this valve is connected via capacitor I to outgoing lead I and directly to outgoing lead E.

The control grid of valve V5 is connected via resistor R16 to lead H which, as described above, is connected via capacitor C3 and potentiometer R4, R5 with the screen grid of valve V3 of receiving circuit SEC.

Line G is normally maintained at negative by negative potential applied through resistor R18.

In the normal unoperated condition of MVC, valve V4 is maintained conducting by the positive potential applied to its control grid via resistor R19 and valve V5 is then non-conducting. The suppressor grid of valve V4 is maintained at this time at a value at which the valve is able to conduct at its anode under the influence of the control grid.

When a negative voltage is applied to the lead D the suppressor grid of valve V4 is also made negative and the valve V4 becomes non-conducting. The anode voltage of this valve rises and outgoing line G becomes positive whereupon, as described above, the mutivibrator is set into operation. When the anode voltage of valve V4 rises a positive impulse is applied to the control grid of valve V5 and this valve conducts over its anode. A negative impulse is applied to outgoing lead I but this impulse performs no function and is ignored. At the same time the voltage of outgoing lead E falls.

Themultivibrator MVB having been set in operation a series of negative impulses appear on line H at times 2:10 ms., t=30 ms. t= ms. but as the relaxation period of the circuit MVC is just over 120 milliseconds as determined by resistor-Rll9 and capacitor C6, the negative impulses applied to the control grid of valve V5 by lead H have no effect. After 120 milliseconds however the negative impulse on lead H and the control grid of valve V5 is able to cause this valve to become non-conducting. The anode voltage of valve V5 rises and a positive impulse applied to the control grid of valve V4. Should the voltage on lead D have risen the valve V4 conducts and the voltage on lead G falls so that the suppressor grid of valve V7 of the multivibrator is made negative and the multivibrator is stopped.

When the valve V5 becomes non-conducting at time t=130 milliseconds a positive impulse is applied to lead I whereupon, as will be described, a stop-signal circuit SSC isactuated to apply a positive impulse on lead N and cause the relay OPR to set the mark and apply a stop signal to the outgoing line Y as has already been described.

When the valve V5 was caused to conduct by the application of'negative potential to lead D to make valve -7 V4 non-conducting, the voltage of, lead E. connected to the screen grid of valve V5 fell and this fall in voltage is supplied to a start-delay circuit.SD.C in order to render this circuit ineffective while the-mutivibrator MVB is in operation.

The potential of the lead D is determined. by a startgate and stop suppressor circuit SGSS which comprises two valves V9-and V14 Lead Dis connected to-a potential divider R R in the anode circuit ofvalveVll).

The incoming line X branches, as de'scribed'above, and one branch A is divided into two sub-branches Q and P. Sub-branch Q is connected, via resistor R28, to the control grid of valve V9. The suppressor grid of this valve is connected to earth via resistor R40. The. anode of valve V9 is connected, via capacitor C10, to the control grid of valve V10 while positive potential is, appliedto this control grid via resistor R27. The anode of valve V9 is also connected, via potential divider, R R to negative potential so that lead: B connected to this' divider is, when the incoming signal on sub-branch Q is mark and valve V9 is cut-off at itscontrol grid, slightly above earth. When the incoming signal is space the potentialof lead B is sufficiently negative to bias the-suppressor grid of valve V11 of start delay circuitSDC to make valve V11 non-conducting over its anode.

Lead Q is also connected to the suppressor grid of valve V10 via resistor R33 so that when the incoming signal is mark valve V10 is non-conducting over its anode and prepared to conduct when the incoming signal is space.

When a space signal appears on the incomingline X and sub-branch Q, the potential of the. control grid of valve V9 is raised and this valveconducts. The consequent drop in the voltage of. the anode of valve V9 is communicated to the control grid of valve V10 which thereupon becomes non-conducting sothat theanode current is still cut 011? although the suppressor grid'of valve V10 has now become positive by virtue of its connection to lead Q. At the same time, as already described, the potential of lead B falls and valve V11 ceases to conduct. The resulting rise in the anode potential of valve V11 is commurn'cated'via resistor R23'to the control grid of valve V12, these valves VII" and V12 forming a pulse trigger circuit. The control grid of valve' V11 is connected to positive voltage source via resistonRZS and via capacitor C8'to the anode of valve V12; The control grid of valve V12 is connected to negative voltage source via resistor R24. The connecting resistor network R20, R21, R23, R24 is so proportioned that, when the potentials applied over either of' the incoming leads E or F are at thelower-value of theirpotential excursions, the potential of the control grid of valve V12 cannotrise above the value. at which the valve V12 is held non-conducting by itscontrol grid. Lead E, as has been described, is connected to thescreen grid of valve. V5 of the. timing circuit MVC and, before. this circuit has, been caused to set,the multi-vibrator MVB in operation, the potential on lead E.is at its higher value. At the same time, i. e. before the multivibrator MVB has been set in operation, the potential on lead F is also at its higher value. In these conditionsof leadsE and F the potential on the control grid of valve V12 rise to cause valve V12 to conduct so that valve V11 becomes non-conducting.

The period of relaxation of the trigger circuit SDC, which period is controlled by the values of resistor R25 and capacitor C8, is adjusted to 10 milliseconds. If, now, a start-signal has appeared on sub-branch Q and valve V11 has been cut-off, the removalof the voltage applied by valve V12 in a direction to maintain the valve V11 cutofi will take place after the start signal has persisted for at least 10 milliseconds. When. this periodhas elapsed valve V11 istallowed to conduct over its screen grid and a brief negative; pulse is. consequently applied to lead C over capacitor C190. The potential of the suppressor gridof valve V9 is consequently loweredifor a short period and valvev V9 is cut-off over its anode. The consequent brief rise. in. the anode potential of valve V9 causes the potential of the control grid of valve V10 to rise and valve,V10. conducts. The resulting fall in the potential of lead D. cuts-ofi valve V4- and the multi-vibrator is set in. operation as described above. If the space signal persists onthe incoming line X the valve V10 reverts to its nonconducting condition. but the consequent rise in. the potential of. lead. D has no effect on valve V4 which is now held non-conducting by its control grid due to the natural periodof V4,.V5.

if the. start signal on the line X does not persist for 10 milliseconds the negative potential which now appears on sub-branch Q is applied to the suppressor grid of valve V10. which. cannot then. become conducting and the potential of line D cannot fall. Valve V9 also becomes non-conducting as the voltage is also applied to its grid. The negative potential is also connected via subbranch P, to the suppressor grid of valve V12 which is thus made non-conducting when the start signal is shorter than 10 milliseconds and consequently valve V11 becomes conducting. A very slight delay which, due to the capacitor C11, occurs before the valve V11 becomes conducting is suffi'cient for the valves V9 and V10 to be rendered non-conducting before a negative impulse can be caused to appear, at lead D. Should, however, a fresh start signal-immediately follow the spurious short start signal it will be appreciated that the circuits S688 and SDC will immediately respond to the new signal and as the circuit MVC has not been afiected by the short start signal no delay will occur in setting the multi-vibrator into operation should the fresh signal persist for at least 10 milliseconds.

When a character has been retransmitted to the outgoingline Y and, at time t=l30 milliseconds, the timing circuit MVC receives a negative pulse over lead H to make valve V5 non-conducting, a positive pulse is transmitted over lead I to the stop-signal circuit SSC. This circuit comprises two multi-grid (pentode) valves V13 and V14 arranged in a pulse trigger circuit with the anode of valve V13 connected to the control grid of valve V14 and the anode. ofvalve V14 connected via capacitor C9 to the control grid of valve V13. The control grid of valve V13 is also connected to positive voltage supply (H. T. positive) via resistor R26 while by a suitable resistance network to which lead I is connected the voltage of the control grid of valve V14 is held at a value at which this valve does not conduct. The suppressor grid'of valve V14 is connected to cathode and earth as is the suppressor grid of valve V13. Lead F is connected to the screen grid of valve V14. Lead N is connected to the anode of valve V13. In the non-operated condition of the stopesignal circuit valve V13 is conducting and valve V14 non-conducting. When, at time t= milliseconds, a positive impulse is transmitted over lead I, valve V14 is caused to conduct and valve V13 therefore, due to the connection of its control grid with the anode of valve V14, becomes non-conducting. A positive impulse is therefore transmitted over lead N to circuit OTC and mark is applied to outgoing line Y. At the same time, i. e. when valve V14 starts to conduct the voltage on lead ,F. falls so that the control grid of valve V12 of the start-delay circuit SDC is lowered and valve V12 is prevented-from conducting while the valve V14 conducts. The start delay circuit is therefore prevented from operating while the stop signal circuit is operated.

The. period of relaxation of the stop-signal circuit SSC is controlled, by the values of resistor R26 and capacitor C9, to be a suitable value bearing in mind that in the repeater now described, the minimum length of the stop signal in a retransmitted character signal is equal to the sum of the periods of the start delay signal and the stopsignal circuit. In the case of 7 unit code signals at a speed of 50 bands, the minimum length is preferably equal to a unit element i; e. 20 milliseconds so that the period of the stop-signal circuit would be 10 milliseconds, the period of the start delay circuit having been determined to be 10 milliseconds. If 7 unit code transmission is employed, the minimum stop signal length might be 0.7 to 0.8 unit i. e. 14 to 16 milliseconds as a compromise between allowing for speed variation and distortion and yet not retransmitting a stop signal unduly shorter than one unit in length. The minimum length of stop signal determines the minimum length of retransmitted character i. e. the minimum spacing in time of the commencement of successive retransmitted start signals. It is evident that this must be no greater than the average spacing in time of the commencement of successive received start signals, otherwise characters would arrive at the repeater at a greater average rate than they can be disposed of or retransmitted. If, for example, a maximum speed error of the transmitting machine of 2 percent must be allowed for, the average spacing of received start signals may be as low as 6.86 units. Even assuming zero error in the timing accuracy of the repeater, the minimum length of retransmitted stop signal must in this case not be greater than 0.85 unit and in practice it may be advisable to adopt a slightly lower limit. In the example now being described the period of the stop-signal circuit SSC would be 4 or 6 milliseconds.

As already described, when the multivibrator MVB is set in operation the voltage of lead E which is connected to the screen grid of valve V is lowered so that the voltage of the control grid of valve V12 is lowered to prevent the start delay circuit from operating when the multivibrator MVB is operaitng i. e. when a character is being retransmitted over line Y. In order to absorb any very short pulses which may occur when the timing circuit MVC is being restored and while this circuit is starting the stop-signal circuit SSC, a capacitor C7 is provided between earth and the potential divider R21, R22.

Should a long space signal be received over the incoming line X, the start-gate and stop signal suppressor circuit SGSS operates as described above for the case when a start signal of normal length i. e. over milliseconds, is receiver. After the short negative pulse has been transmitted over lead D to start the multivibrator MVB valve V9 continues to conduct over its anode and valve V10 is held non-conducting. The time constant of capacitor C10 and resistor R27 is arranged to be such that unless the space signal is of a greater length then 110 milliseconds, valve V10 will not conduct under the infiuence of the incoming line condition so long as this valve is cut-ofi at its control grid by the coupling with the anode of valve V9 and at the suppressor grid by its connection with sub-branch Q.

The time constant of C10, R27 is, moreover, made such that valve V10 will conduct before time t=130 milliseconds. When, therefore, a long space signal appears on the line X the valve V10 will, at a determined time in advance of t=130 milliseconds, conduct and cause the potential of lead D to fall thus holding the suppressor grid of valve V4 of the timing circuit negative and preventing valve V5 from ceasing to conduct at that instant. A positive pulse can therefore not be transmitted over lead I to the stop-signal circuit so that a stop signal is prevented from being applied to the line Y. This condition persists until the incoming line X reverts to mark when V10 becomes cut-off at the suppressor grid and the timing circuit MVC is allowed to restore to its non-operated condition and the stop-signal circuit SSC is triggered to cause mark to be applied to line Y.

With the supply and signal voltages specified in the foregoing description, suitable valves and component values are given in the following table by way of example:

Circuit Component Type or Value pgggwawaa SEC OTC R MVB MVO

SGSS

SDO

We claim:

1. An electronic regenerative telegraph repeater comprising an incoming signal line and an outgoing signal line, a relay for applying to the outgoing line signals of polarity corresponding to signals received on the incoming line, a receiving circuit to which the incoming signals are applied and which is connected through an electronic switching circuit to the relay, a multivibrator circuit arranged to apply to the receiving circuit conditioning impulses related in frequency to a desired speed of signal transmission, a timing circuit arranged to render the multivibrator operative immediately a start signal of predetermined duration is received and a start delay circuit which prevents the timing circuit from rendering the multivibrator circuit operative until a start signal has persisted for a predetermined minimum period.

2. An electronic regenerative repeater according to claim 1, in which the cessation of a start signal of less than the predetermined period causes the start delay circuit to be reset and conditioned to accept a new start signal substantially immediately.

3. An electronic regenerative telegraph repeater according to claim 1, in which the receiving circuit is arranged on completion of a predetermined number of impulses to apply an impulse to the timing circuit and to cause the timing circuit to stop the multivibrator circuit.

4. An electronic regenerative telegraph repeater according to claim 1, in which the receiving circuit includes a multi-grid valve and the electronic switching circuit includes a pair of valves having the anode of one connected to the control grid of the other and in which the screen grid of the multi-grid valve is coupled to one valve in said pair, the control grid of the multi-grid valve is arranged to receive impulses from the multivibrator circuit and the anode of the multi-grid valve is coupled to the other of said pair of valves, the arrangement being such that impulses produced by the multi-grid valve control the operation of the electronic switching circuit.

5. An electronic regenerative telegraph repeater according to claim 4, in which the incoming signal line is connected to the suppressor grid of the multi-grid valve.

6. An electronic regenerative telegraph repeater according to claim 1, including a circuit for providing a stop signal connected to the electronic switching circuit and which applies a stop signal for a predetermined limited period.

7. An electronic regenerative telegraph repeater according to claim 6, in which the stop signal circuit is arranged to prevent the start delay circuit from operating to permit the multivibrator circuit to be set in operation during the operated period of the stop signal circuit.

8. An electronic regenerative telegraph repeater according to claim 1, in whichthe timing circuit is arranged to render the start delay circuit inoperative while the multivibrator circuit is in operation.

9. An electronic regenerative telegraph repeater according to claim 6, including a stop signal suppressor circuit arranged to render the stop signal circuit inoperative to apply a stop signal to the outgoing line while a long space signal exists on the incoming signal line.

10. An electronic regenerative telegraph repeater according to claim 9 in which the stop signal suppressor circuit comprises also a switching circuit through which a start signal on the incoming signal line is communicated to the start delay circuit.

References Cited in the file of this patent UNITED STATES PATENTS 2,406,096 Morrison Aug. 20, 1946 2,430,547 Anderson Nov. 11, 1947 2,474,490 Pelle June 28, 1949 2,502,943 Goodall Apr. 4, 1950 FOREIGN PATENTS 470,654 Great Britain Aug. 19, 1937 

